Nitration process is used for the production of many large-volume chemicals such as nitrotoluenes, nitrochlorobenzenes and other nitroarenes which are vital intermediates for dyes, pharmaceuticals, perfumes and pesticides. Nitration of arenes is performed classically with a mixture of nitric and sulfuric acids. Major disadvantages of this method are formation of by-products of polynitration and environmental pollution during disposal of spent acid.
Reference may be made to a US patent (U.S. Pat. No. 4,418,230 November 1983) wherein a method for nitration of toluene with HNO.sub.3 and H.sub.2 SO.sub.4 at 0-50.degree. C. is described (o:m:p 55:2.3:38.9). The drawbacks are the use of sulfuric acid and also the formation of more of o-isomer.
Reference may be made to another US patent (U.S. Pat. No. 1,12,006 September 5) wherein a process for nitration of toluene with HNO.sub.3 and 10% H.sub.2 SO.sub.4 impregnated Al.sub.2 O.sub.3 and 1% Mo at 135-145.degree. C./20 Torr is described (o:m:p 34.0:3.5:62.5). The drawbacks are using of sulfuric acid in the preparation of the catalyst and the catalyst gets deactivated on each cycle.
Although it has been known for some time that benzene and its homologs can be nitrated with HNO.sub.3 alone without using sulfuric acid, little or no progress has been made in this direction on commercial scale. The disadvantage in this method is the low productivity and the use of large excess of nitric acid (molar ratio of nitric acid to benzene is 2:1 to 4:1) which increase the possibility of poly-nitro compounds formation and affect the economics of the project. Recently attention has been focused on the development of environmentally friendly solid acid catalysts such as Nafion especially in Friedel-Crafts reactions to replace environmentally hazardous sulfuric acid in nitration reactions.
Reference may be made to a US patent (U.S. Pat. No. 4,234,470; Nov. 18, 1980) wherein a method for nitration of benzene, chlorobenzene and toluene with HNO.sub.3 in presence of Nafion catalyst is described. The drawbacks are the use of expensive Nafion resin whose activity is decreasing on each cycle and offers nearly identical isomeric selectivity (o:m:p 56:4:40) as that of mixed acid.
Reference may also be made to a Japanese patent (Jpn. Kokai Tokkyo Koho JP 63, 303, 957, June, 1987) wherein a process for vapour phase nitration of halobenzenes with HNO.sub.3 and aluminosilicates at 175.degree. C. and space velocity 2.2 h.sup.-1 (nitration of chlorobenzene isomeric ratio: o:m:p 14.5:1.7:83.8) is described. The draw backs are that the reactions are carried in vapour phase conditions. Reference may also be made to an Indian patent (Appl., NS90/97 2937 Del 97 October 1997) and an European patent (Appl., 98302258. 3-1521, Mar. 25, 1998) wherein a process for the production of nitroarenes from arenes with fuming HNO.sub.3 in presence of exchanged clays as catalysts is described. The draw backs are that this invention does not significantly alter isomeric composition of disubstituted benzenes.
Reference may also be made to a publication (Chem.Commun, 469, 1996) wherein benzene, alkylbenzenes and halobenzenes are nitrated in quantitative yields and with high para-selectivity in a solvent free process by use of a stoichiometric quantity of nitric acid and acetic anhydride at 0-20.degree. C. in the presence of zeolite beta as a recyclable catalyst. Reference may be also made to a publication (Catal.Lett., 255, 1997) wherein halobenzenes are nitrated in quantitative yields and with high para-selectivity in CCl.sub.4 solvent by using stoichiometric quantities of nitric acid and acetic anhydride at room temperature and 70.degree. C. in the presence of sulphated zirconica (SO.sub.4.sup.2- /ZrO.sub.2) as catalyst. Reference may also be made to publication (J. Mol. Catal., 87, 33, 1994) wherein halobenzenes are nitrated in good yields and with high para-selectivity in CCl.sub.4 solvent by use of a stoichiometric quantity of nitric acid and acetic anhydride at room temperature to 80.degree. C. in the presence of Fe.sup.3+ -montmorillonite catalyst. The draw backs are the use of expensive acetic anhydride in all the above processes which is uneconomical and also exothermic and explosive nature of the reaction of acetic anhydride and nitric acid.
Reference may also be made to a publication (Chem. Commun., 613, 1997) wherein aromatics are nitrated in good yields in dichloroethane solvent by using stoichiometric quantities of 69% nitric acid at reflux temperature of solvent in the presence of lanthanide(III) triflates as recyclable catalysts. The draw backs are the slowness of the reaction which takes more time for completion of the reaction, the use of expensive catalyst and the process will be of low productivity. The isomeric ratio is also identical with that of mixed acids.
The main objective of the present invention is to provide a process for the production of nitroarenes with high para-selectivity from monosubstituted aromatic hydrocarbons using aluminosilicates as catalysts which obviates the drawbacks as detailed above. Another object of the present invention is the use of ecofriendly modified aluminosilicates as solid acid catalysts in the nitration of monosubstituted arenes dispensing the use of sulfuric acid which obviates the drawbacks as detailed above.
Yet another object of the present invention is to dispense with the use of acetic anhydride which obviates the drawbacks as detailed above.